The present invention relates to signature recognition devices and in particular to a writing stylus sensitive to writing pressure.
Various devices exist for verifying a person's identity through indicia unique to that individual. Common indicia include fingerprints, voice graphs, retina scans and signature verification. Comparison of a sample signature with a reference, or valid, signature is a practical and efficient method of identity verification. Each individual signature has characteristics unique to that signature. These characteristics can be measured and used to discern a forged signature from a genuine signature.
Measurable signature characteristics include pen pressure as a function of a dynamic such as time, pen direction and pen velocity. Of these characteristics, pen pressure is the most commonly measured characteristic. Pressure can be detected either on the writing surface or on the writing tool itself. Measuring the pressure on the writing tool proves to be the more convenient of these two choices, since instrumentation need not be provided at the location of every possible writing surface.
Some pressure-sensitive writing tools are known in the art. One such tool is described in U.S. Pat. No. 4,896,543. In the device of the '543 patent, a three-axis pressure measurement stylus contains a plurality of pressure-sensitive mylar sheets located adjacent to the tip of the device. However, for the pressure-sensitive writing tool to be accurate and reliable, the pressure measurement should not depend on the indirect measurement of pressure by sensing strain, as with a strain gage, since the modulus and dimensional accuracy of the material used are subject to manufacturing variations and environmental influences. The pen assembly should preferably measure writing pressures directly, without significant compliance between the pen tip and pen housing. Systems depending on spring deflection provide unwanted compliance.
Other writing tools disclosed in the prior art also contain measuring hardware which is coupled to the tip or to the ink shaft of the tip. The device disclosed in U.S. Pat. No. 4,646,351 to Asbo et al. and in U.S. Pat. No. Re 29,765 to Crane et al. each contain a set of strain gages located on a suspended flexible diaphragm which is disposed on the shaft of the stylus. The strain gages measure the flexure of the tip shaft.
In addition, U.S. Pat. No. 4,111,052 shows strain measurement in a bending tube using separate strain gages. The use of separate strain gages requires special additional structures in order to obtain bending parameter inputs. U.S. Pat. No. 4,078,226, on the other hand, shows the measurement of deflection along long PZT beams, wherein the bending of the pen cartridge is measured, rather than the stress in the cartridge itself.
It is useful to note that it is known to use selective metallization of PZT structures to provide selective outputs. See for example U.S. Pat. No. 4,513,437.
What is needed is a stylus suited to detecting applied pressures that is substantially immune to damage from the inadvertent application of excessive pressures and that is capable of accurate measurements of strain without impediments due to the mechanical and environmental characteristics of the carrier structure.